1. Field of the Invention
This invention relates to an electronic sensor, and, in particular, to a sensor suitable for detecting the location of an object behind a variety of surfaces, including walls, floors and other non-electrically conductive structures. More specifically, the invention relates to an electronic sensor used to detect centerlines and edges of wall studs, floor joists, and the like.
2. Description of the Prior Art
U.S. Pat. No. 4,464,622 entitled “Electronic wall stud sensor,” issued Aug. 7, 1984, and incorporated in its entirety by reference herein, discloses an electronic wall stud sensor particularly suitable for locating a stud positioned behind a wall surface. (A “stud” is a structural member of a building to which an interior wall surface such as wall board or paneling is affixed.) Typically in the U.S., “2-by-4” wooden studs are used in construction. Nominally, a 2-by-4 stud is 51 mm (2 inches) wide and 102 mm (4 inches) deep and of any suitable length. The actual dimensions of a 2-by-4 are more typically 38 mm (1½ inches) wide and 89 mm (3½ inches) deep. Use of English (inches) units and U.S. stud sizes here is in conformance with U.S. construction practice and is not intended to be limiting, but is only illustrative. Finding studs is a typical problem for building repairs, picture hanging, etc.
The sensor detects the stud by measuring a change in capacitance due to a change in the dielectric constant along the wall. Due to the placement of the studs, a wall surface exhibits changing dielectric constants while the sensor is moved along the wall surface. The sensor includes a plurality of capacitor plates, a circuit for detecting changes in the capacitance, and an indicator.
The plurality of capacitor plates is mounted in the sensor such that they can be positioned close to a wall's surface. When the capacitor plates are drawn along the surface, the circuit detects a change in the capacitance of the plates due to a change in the average dielectric constant of the surface. The capacitor plates are used to measure the effective capacitance or change in capacitance of a wall. Before detection begins, the sensor first performs a calibration to null out the effect of a wall in the absence of a stud. The capacitor plates are composed of a center plate and a symmetric pair of electrically connected edge plates. The difference in capacitance between the center and edge plates is used to determine the location of the edge of a stud. The centerline of the stud is then determined by finding both the left and right edges of the stud and then measuring to the middle of the distance between the edges. Thus, multiple measurements must be made in order to determine the centerline of the stud. The indicator indicates the change in capacitance of the capacitor plate, thereby alerting an operator to the wall stud position. The indicator also alerts the operator when calibration is occurring.
While this procedure is effective in determining the centerline of a stud, significant errors in determining the location of the stud's edges can occur. One factor is the depth of the stud behind the surface. Due to the thickness of the sheetrock (also referred to as gypsum wall board and which has a thickness of 16 mm or equivalently ⅝ of an inch) or other wall surface material, a “ballooning” effect may distort the perceived width of the stud. The closer a stud is positioned to the surface, the wider the stud will appear when sensed in this way. Similarly, the farther or deeper a stud is positioned, the narrower the stud will appear. This ballooning effect is exacerbated when the sensitivity of the sensor is increased to aid in detecting deeper studs. The ballooning may be asymmetric due to electrical wires, metallic pipes and other objects in close proximity to the stud, which in turn may lead to a reduced the ability to accurately determine a stud's centerline. In the case of extreme ballooning, location of an edge of a stud can be inaccurately indicated by as much as 51 mm (2 inches). Similarly, the centerline of the stud may be so inaccurately indicated that it is completely off the actual stud location.
A first method of compensating for the ballooning effect is shown in U.S. Pat. No. 6,023,159, entitled “Stud sensor with dual sensitivity,” issued Feb. 8, 2000, and incorporated by reference herein in its entirety. Unfortunately, using a dual sensitivity control only partially minimizes the ballooning effect.
A second method of compensating for the ballooning effect is shown in U.S. Pat. No. 5,917,314, entitled “Electronic wall-stud sensor with three capacitive elements,” issued Jun. 29, 1999, and incorporated by reference herein. This second method discloses using three parallel sensing plates and using sums and differences between the various plate capacitances to determine the centerline and edges of a stud.
The above methods, which use electronic wall stud sensors, are unable to reliably and accurately sense an edge of a stud (or other structural member) through surfaces that are thicker than 38 mm (1½ inches). Additionally, these sensors, if overly sensitive, falsely indicate the presence of non-existing studs. Therefore, known sensors have disadvantages.